In the prior art, an arc-shaped magnet used as a field magnet for a small DC motor has been formed mainly of ferrite sintered magnet and ferrite bond magnet, and partly of rare earth-iron bond magnet manufactured by extrusion molding.
Generally, an arc-shaped magnet used as a field magnet is disposed outside of an armature for a small DC motor. A thickness of conventional arc-shaped magnet was not less than 1 mm, no magnet having a thickness of less than 1 mm has been available. When a motor is down-sized, therefore, a motor can not maintain output power since a diameter of an armature must be made smaller. Especially, in case of ferrite magnet, whether it is sintered or bonded, output power of a motor shows remarkable decrease due to the downsizing because enough static magnetic field is not applied to a gap between a field magnet and an armature.
Therefore, a thin-arc-shaped rare earth-iron bond magnet is awaited to provide enough static magnetic field to a gap between an armature and the bond magnet in a small motor.
For down-sizing motors, however, following problems must be solved to manufacture a rare-earth arc-shaped magnet having a wall thickness of less than 1 mm used as a field magnet in a small DC motor.
(1) Poor toughness of a sintered magnet causes breakage or cracks easily. A magnet less than 1 mm thickness is, therefore, hard for being integrated into a motor.
(2) To produce a magnet by injection molding, a molding die cavity must be filled with injected magnetic compound composed of magnet powder and thermoplastic resin. It is difficult, however, to inject magnetic compound having much amount of magnet powder into a molding die cavity of less than 1 mm width.
(3) To produce a magnet by powder molding, a compressed powder is formed first, using magnetic compound composed of magnet powder and thermosetting resin, then heated to cure the compressed powder. It is difficult, however, in powder molding, to fill molding die cavity with the magnetic compound uniformly. Moreover, molding die itself would be damaged due to such a cavity of less than 1 mm width.
(4) To produce a magnet by extrusion molding, extruded magnetic compound composed of magnet powder and thermoplastic resin is cooled just after extruded from a molding die head in case of a magnet of less than 1 mm thickness, which easily causes deformation in the magnet. Therefore, it is hard to produce a magnet having a wall thickness of less than 1 mm and having dimensional accuracy which allows the magnet to be directly integrated into a motor.
(5) As a method for producing an arc shaped magnet thinner than 1 mm, a machining into less than 1 mm from more than 1 mm thickness may be a solution for a magnet originally formed by injection molding, powder molding or extrusion molding as above mentioned way through 2 to 4. However, machining will easily cause fine cracks, which will make it difficult to integrate the magnet into a motor like the case 1 of a sintered magnet.
Consequently, for the above mentioned thin-arc-shaped magnet, a method of suppressing deformation of magnet is shown in Japanese Patent Non-examined Laid-Open Publication No. H06-236807, disclosing a method of manufacturing comprising the steps of:                (1) filling a molding die with molten magnetic compound composed of magnet powder and thermoplastic resin; and        (2) extruding magnetic compound while being cooled the temperature in the molding die below the melting point of thermoplastic resin to prevent deformation.        
According to the method disclosed, for example, arc-shaped magnet having a thickness of 0.9 mm can be produced by extrusion molding with a deviation of ±30 μm using magnetic material composed of 95 wt. % of rare earth-iron melt-spun Nd—Fe—B alloy flakes and thermoplastic resin mainly composed of 12-nylon.
In this way, however, magnet characteristics decrease accordingly, compared with a compression-molded magnet, due to reduced volume of magnet powder included because molten thermoplastic resin must act as a carrier of magnetic compound. Furthermore, magnetic material in molding die must be cooled below the melting point of thermoplastics of magnetic compound and solidified in order to form an arc-shaped magnet thinner than 1 mm with dimensional accuracy of not more than 30 μm in thickness deviation. Therefore, the method has drawbacks of molding-die-wear and increase in energy consumption because stronger extruding force is required and extruding speed decreases.
Generally, so-called powder molded magnet includes 1.5–3.0 wt. % of resin and has magnet density of 5.9–6.1 g/cm3. The magnet is produced using rare earth-iron melt-spun flakes (for example, true-density: 7.55 g/cm3) and thermosetting resin (for example, epoxy resin true density: approx. 1.15 g/cm3) to form a compressed powder by powder molding, followed by curing by heating thermosetting resin.
On the other hand, magnet density of injection molded or extrusion molded bond magnet is, generally, less than 5.7 g/cm3, since much amount of thermoplastics (for example, 12-nylon true density: about 1.1 g/cm3), at least 5 wt. %, is needed, which decreases magnet density. A magnetic characteristics of such a magnet is not preferable for a small DC motor to apply a powerful static magnetic field in the gap between an armature and a field magnet, compared with powder-molded magnet, since the characteristics depends only on magnet density.
Therefore, thin-arc-shaped powder molded magnet, with its higher magnet density and dimensional accuracy, has been demanded for a high power permanent magnet motor.